wsk activation poll <id>

@rabbah commented on [Sat Sep 03 2016](https://github.com/apache/incubator-openwhisk/issues/1201)

We currently implement `wsk activation poll` as a console view that shows all new activations. It is sometimes useful to implement `wsk activation poll ` to wait for a particular activation and retrieve its result.

This could be useful as a standalone command or as part of `wsk action invoke --blocking` to retrieve the action result for long actions (more than 1 minute).